Steel parts such as a gear and a bearing are used in a severe environment where they are subjected to a large load by transmission of torque or the like. For this reason, high fatigue strength and abrasion resistance are required for the above-described steel parts. After being formed into a shape to be used, these steel parts are subjected to a surface hardening treatment to have the required high fatigue strength and abrasion resistance provided thereto while securing the internal toughness.
Si in steel exhibits high resistance to temper softening in high carbon martensite, so that for achieving an increase in strength of surface fatigue strength, the content of Si is desirably increased. For example, in Patent Document 1, there has been disclosed a technique in which the content of Si in steel is set to 0.5 to 3.0% and vacuum carburizing is performed. However, the vacuum carburizing has disadvantages in that a continuous treatment is difficult to be performed, tarring occurs, it is difficult to control part properties, and so on, and thus it is difficult to achieve mass production.
In contrast to this, gas carburizing does not have these disadvantages, and as a surface hardening treatment intended for mass production, the gas carburizing is more preferable than the vacuum carburizing.
However, Si in the steel deteriorates carburizing performance in the gas carburizing. The deterioration of carburizing performance means that under the same carburizing condition, the depth of a hardened layer obtained by the carburizing is inferior to that of a case hardened steel used normally such as a JIS steel SCr420.
For example, Non-Patent Document 1 has reported that with an increase in content of Si, a gas-carburized depth is decreased, and the upper limit of the content of Si to which gas carburizing is applicable is 1.2%. For this reason, the development of a technique making gas carburizing possible on a high Si-containing steel has been desired.